The invention relates to the design and manufacture of integrated circuits, and more particularly, to techniques, systems, and methods for implementing metal-fill on an integrated circuit.
In recent years, in integrated circuit (IC) manufacturing, chemical-mechanical polishing (CMP) has emerged as an important technique for planarizing dielectrics because of its effectiveness in reducing local step height and achieving a measure of global planarization not normally possible with spin-on and resist etch back techniques. However, CMP processes have been hampered by layout pattern dependent variation in the inter-level dielectric (ILD) thickness which can reduce yield and impact circuit performance.
Metal-fill patterning is a common approach for reducing layout pattern dependent dielectric thickness variation. Metal-fill patterning is the process of filling large open areas on each metal layer with a metal pattern to compensate for pattern-driven variations. The manufacturer of the chip normally specifies a minimum and maximum range of metal that should be present at each portion of the die. If there is an insufficient amount of metal at a particular portion on the chip, then metal-fill is used to increase the proportion of metal in that portion. However, too much metal may cause dishing to occur. Therefore, the metal-fill process should not cause the die to exceed any specified maximum range of metal for the chip.
The addition of the metal fill may negatively affect the electrical performance of the integrated circuit device. In particular, placement of metal fill shapes near existing features on the IC may cause cross coupling capacitances to occur on the device. As timing closure is a critical goal in the design of high speed devices, additional cross coupling capacitances can cause additional iterations in the design process due to slowing down some signals.
One solution to address the problem is to insert fewer metal fill geometries, which would result in less cross coupling capacitance. This approach causes a wider variation in the metal density percentage in the windows across the device as some areas already meet the user's preferred target density without adding any metal fill shapes at all. The problem with that lack of consistency is that there will be more OCV (On Chip Variation) issues as the wiring in higher density areas will become thinner than in the lower density areas. This difference in thickness is very difficult to account for.
Another solution is to place metal fill farther away from existing objects in the layout. A metal fill tool may use spacing rules when inserting metal fill, which mandate that metal fill cannot be inserted within a specified distance around existing shapes. These rules are uniformly applied for all shapes in the design. In an attempt to avoid harmful capacitance effects, organizations may increase the mandated spacing distance so that the actual spacing distance between metal fill and existing shapes exceeds what is required by the spacing rules. For instance, if the spacing rules require a distance of x, then organizations may actually use a spacing distance of 2× to avoid timing problems. The problem is that this approach of trying to put extra space around every object will significantly restrict the free space available to place metal fill, thereby preventing metal fill tools from hitting density targets for the fill.
Therefore, there is a need for an improved approach for implementing metal fill on an electrical device without causing creating cross-coupling capacitance problems. In some embodiments of the invention, timing aware metal fill insertion is performed to avoid or minimize cross-capacitance problems on the IC design. A cost may be assigned to different candidate metal fill shapes. The cost is associated with the expected effect upon timing requirements by the metal fill shape, with lower costs corresponding to lower expected impacts upon the timing requirements. To meet density requirements, lower cost metal fill shapes are inserted prior to higher cost metal fill shapes.
Further details of aspects, objects, and advantages of the invention are described below in the detailed description, drawings, and claims. Both the foregoing general description and the following detailed description are exemplary and explanatory, and are not intended to be limiting as to the scope of the invention.